gRPC中，Server、Client共享的Class不是很多，所以我们可以单独的分别讲解Server和Client的源码。
通过第一篇，我们知道对于gRPC来说，建立Server是非常简单的，还记得怎么写的？还是以example里 HelloWorldServer 例子来看
server = ServerBuilder.forPort(port)
.addService(new GreeterImpl())
.build()
.start();
你没有看错，就是这么几行搞定。
如果需要看懂gRPC的源码，首先有几点需要明白
Builder模式生成Entity
Provider（SPI）模式解耦，动态选择服务提供方
abstract class用于扩展
0. 流程图
1. Builder
ServerBuilder是一个抽象类，不同的服务提供方（Provider），将继承实现它。如何找到这些继承者呢？ServerProvider就是用来找到不同的provider的。
2. Provider
如上图，ServerProvider也是一个抽象类，实现者都有哪些呢？我们通过SPI模式找到他们。
通过搜索文件知道gRPC中 io.grpc.ServerProvider 的实现方只有：Netty
io.grpc.netty.NettyServerProvider，
这个类就是ServerProvider的实现者，它的builderForPort返回ServerBuilder
3. NettyServer
最后，我们来看下当链接建立时是如何创建handle的。
public void initChannel(Channel ch) throws Exception {
eventLoopReferenceCounter.retain();
ch.closeFuture().addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) {
eventLoopReferenceCounter.release();
}
});
NettyServerTransport transport = new NettyServerTransport(ch, protocolNegotiator,
maxStreamsPerConnection, flowControlWindow, maxMessageSize, maxHeaderListSize);
ServerTransportListener transportListener;
// This is to order callbacks on the listener, not to guard access to channel.
synchronized (NettyServer.this) {
if (channel != null && !channel.isOpen()) {
// Server already shutdown.
ch.close();
return;
}
transportListener = listener.transportCreated(transport);
}
transport.start(transportListener);
}
看code可知，当一个链接建立时，会生成一个NettyServerTransport，所有的数据处理都将在这里实现。
4. NettyServerTransport
public void start(ServerTransportListener listener) {
Preconditions.checkState(this.listener == null, "Handler already registered");
this.listener = listener;
// Create the Netty handler for the pipeline.
final NettyServerHandler grpcHandler = createHandler(listener);
HandlerSettings.setAutoWindow(grpcHandler);
// Notify when the channel closes.
channel.closeFuture().addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
notifyTerminated(grpcHandler.connectionError());
}
});
ChannelHandler negotiationHandler = protocolNegotiator.newHandler(grpcHandler);
channel.pipeline().addLast(negotiationHandler);
}
我们看到当调用start方法是，最重要的就是createHandle，在这个方法里将看到如何绑定HTTP/2的处理器的。
5. NettyServerHandle
static NettyServerHandler newHandler(ServerTransportListener transportListener,
int maxStreams,
int flowControlWindow,
int maxHeaderListSize,
int maxMessageSize) {
Preconditions.checkArgument(maxHeaderListSize > 0, "maxHeaderListSize must be positive");
// 就是一个log
Http2FrameLogger frameLogger = new Http2FrameLogger(LogLevel.DEBUG, NettyServerHandler.class);
Http2HeadersDecoder headersDecoder = new GrpcHttp2ServerHeadersDecoder(maxHeaderListSize);
// reader
Http2FrameReader frameReader = new Http2InboundFrameLogger(
new DefaultHttp2FrameReader(headersDecoder), frameLogger);
// writer
Http2FrameWriter frameWriter =
new Http2OutboundFrameLogger(new DefaultHttp2FrameWriter(), frameLogger);
return newHandler(frameReader, frameWriter, transportListener, maxStreams, flowControlWindow,
maxMessageSize);
} @VisibleForTesting
static NettyServerHandler newHandler(Http2FrameReader frameReader, Http2FrameWriter frameWriter,
ServerTransportListener transportListener,
int maxStreams,
int flowControlWindow,
int maxMessageSize) {
Preconditions.checkArgument(maxStreams > 0, "maxStreams must be positive");
Preconditions.checkArgument(flowControlWindow > 0, "flowControlWindow must be positive");
Preconditions.checkArgument(maxMessageSize > 0, "maxMessageSize must be positive");
// 一个channel一个connection
Http2Connection connection = new DefaultHttp2Connection(true); // Create the local flow controller configured to auto-refill the connection window.
connection.local().flowController(
new DefaultHttp2LocalFlowController(connection, DEFAULT_WINDOW_UPDATE_RATIO, true)); Http2ConnectionEncoder encoder = new DefaultHttp2ConnectionEncoder(connection, frameWriter);
Http2ConnectionDecoder decoder = new DefaultHttp2ConnectionDecoder(connection, encoder,
frameReader); Http2Settings settings = new Http2Settings();
settings.initialWindowSize(flowControlWindow);
settings.maxConcurrentStreams(maxStreams); return new NettyServerHandler(transportListener, decoder, encoder, settings, maxMessageSize);
}